PROPAGATION OF ACTION-POTENTIALS IN THE DENDRITES OF NEURONS FROM RATSPINAL-CORD SLICE CULTURES

1. We examined the propagation of action potentials in the dendrites o f ventrally located presumed motoneurons of organotypic rat spinal cor d cultures. Simultaneous patch electrode recordings were made from the dendrites and somata of individual cells. In other experiments we vis ualized the membrane voltage over all the proximal dendrites simultane ously using a voltage-sensitive dye and an array of photodiodes. Calci um imaging was used to measure the dendritic rise in Ca2+ accompanying the propagating action potentials. 2. Spontaneous and evoked action p otentials were recorded using high-resistance patch electrodes with se parations of 30-423 mu m between the somatic and dendritic electrodes. 3. Action potentials recorded in the dendrites varied considerably in amplitude but were larger than would be expected if the dendrites wer e to behave as passive cables (sometimes little or no decrement was se en for distances of >100 mu m). Because the amplitude of the action po tentials in different dendrites was not a simple function of distance from the soma, we suggest that the conductance responsible for the boo sting of the action potential amplitude varied in density from dendrit e to dendrite and possibly along each dendrite. 4. The dendritic actio n potentials were usually smaller and broader and arrived later at the dendritic electrode than at the somatic electrode irrespective of whe ther stimulation occurred at the dendrite or soma or as a result of sp ontaneous synaptic activity. This is clear evidence that the action po tential is initiated at or near the soma and spreads out into the dend rites. The conduction velocity of the propagating action potential was estimated to be 0.5 m/s. 6. The amplitude of the dendritic action pot ential could also be initially reduced more than the somatic action po tential using 1-10 mM QX-314 (an intracellular sodium channel blocker) in the dendritic electrode as the drug diffused from the dendritic el ectrode toward the soma. Furthermore, in some cases the action potenti al elicited by current injection into the dendrite had two components. The first component was blocked by QX-314 in the first few seconds of the diffusion of the blocker. 7. In some cells, an after depolarizing potential (ADP) was more prominent in the dendrite than in the soma. This ADP could be reversibly blocked by 1 mM Ni2+ or by perfusion of a nominally Ca2+-free solution over the soma and dendrites. This sugges ts that the back-propagating action potential caused an influx of Ca2 predominantly in the dendrites. 8. With the use of a voltage-sensitiv e dye (di-8-ANEPPS) and an array of photodiodes, the action potential was tracked along all the proximal dendrites simultaneously. The resul ts confirmed that the action potential propagated actively, in contras t to similarly measured hyperpolarizing pulses that spread passively. There were also indications that the action potential was not uniforml y propagated in all the dendrites, suggesting the possibility that the distribution of Na+ channels over the dendritic membrane is not unifo rm. 9. Calcium imaging with the Ca2+ fluorescent indicator Flue-3 show ed a larger percentage change in fluorescence in the dendrites than in the soma. Both bursts and single action potentials elicited sharp ris es in fluorescence in the proximal dendrites, suggesting that the back -propagating action potential causes a concomitant rise in intracellul ar calcium concentration. This might have important consequences for t he modulation of metabolic processes that in turn might affect the mod ulation of synaptic transmission by the postsynaptic neuron.10. In sum mary, the results indicate that the dendrites of motoneurons have nonu niformly distributed Na+ and Ca2+ conductances. These are activated by the spreading action potentials that are generated at or near the som a and propagate back into the dendrites. This has important consequenc es for the processing of synaptic input by motoneurons as well as for their intrinsic firing properties.